Design and Engineering
Construction of the The Loopline Bridge commenced in September 1889 as a means of connecting the north and south rail lines in the city. The original design carried out by J. Chaloner Smith, Engineer to the Dublin, Wicklow and Wexford Railway Company, in 1873 was modified by him for the purpose of passing Parliament in 1884. The contractor was Arrol Brothers from Scotland.
The bridge itself consists of wrought iron lattice girders on a double row of braced cast iron piers while the viaduct section is located on faux granite white limestone piers. The bridge section which carries two rail lines of sharply curved ballasted track consists of three spans of 38m, 40m and 39m, approximately 6.1m over street level.
The two river piers are braced cast iron cylindrical piers. The two cylindrical members per pier were hollow and constructed as caissons. They were sunk through approximately 3m of river mud and gravel where they are anchored into rock by means of centre dowels which extend 1.5m into the hard layer. The cylinders are in-filled with concrete. The cylinders were constructed in a series of vertical levels, there being six in all. The top three sections are 3m diameter tapering from 4.2m diameter sections. These cylinders are braced together by a central cross-brace and diagonals which are enclosed by concrete and a cast iron surround of 75mm to 50mm gauge. Above high water level the cylinders are cross-braced by a semi-circular arched member with hollow spandrel. The uppermost parts of the cylinders are topped by ornamented sections which carry the main lattice girders.
The spans are simply supported with a fixed bearing at one end and a roller bearing at the other end. The cross-girders originally carried a steel trough deck, however, due to deterioration these were replaced between 1958 and 1960. During this time the cross-girders were replaced or reconditioned by means of an extra top flange plate, and troughing was replaced by a system of steel stringers welded to flat deck plating. Furthermore the original single line of bracing between cross-girders was replaced by two rows of new angle cross- bracing. On the longest span the span/depth ratio is 11 which would indicate a good degree of stiffness.
The construction of this bridge effectively finished the river upstream of this point being navigable for sea going vessels due to the height restriction. The Loopline Bridge has always been contentious because of its proximity to and how it blocked the view of the Custom House as you approach from the city.